Inverters have numerous applications in electrical power supplies including the production of alternating current power supplies, for example, when used as an inverter to convert a DC output voltage into an AC power supply (e.g. an uninterruptible power supply). They may also be used in internal stages of DC to DC converters, induction heating, microwave generation, surface detection, medical experimentation, high frequency radio systems, inductively coupled power transfer (ICPT) systems, etc.
A conventional push-pull current fed resonant inverter is shown in FIG. 1. The operation of such inverters is discussed in U.S. Pat. No. 5,450,305 the contents which are incorporated herein by reference. These resonant inverters have gained much popularity due to their low switching losses and low electromagnetic interference (EMI). A fundamental problem with these inverters is that large magnetic components i.e. inductors and transformers are required. These components are physically large, heavy and expensive, so they limit opportunities to reduce size, weight and cost of these inverters.
For example, in FIG. 1 the inverter needs a “DC” or decoupling inductor Ld which decouples the inverter from the DC power source, providing a current source and allowing the voltage in the resonant circuit to oscillate freely without restraint. The circuit of FIG. 1 also has a phase splitting transformer (represented by inductors Lsp). When this inverter design is used for ICPT systems another magnetic coil or track loop L is needed to couple with secondary power pickups to achieve contactless power transfer. If the circuit is used as a DC to DC converter, then more transformers or secondary windings may be required to provide the DC power output.
The decoupling inductor Ld is required to provide a constant current source under steady state operating conditions. This inductor is usually designed to be large to overcome saturation problems. The phase splitting transformer with the two closely coupled windings Lsp is used to divide the DC current into two branches, and the switches S1 and S2 are controlled to be “on” and “off” alternately, to change the direction of the current that is injected into the resonant tank circuit which comprises the coil L and its tuning capacitor C. The resistor R represents the load supplied by the inverter, and in FIG. 1 also includes the resistance of inductor L.
An external controller (not shown) is also required in order to control the switches S1 and S2. The controller detects the resonant voltage (for example sensing voltage across tuning capacitor C) and drives the switches at zero voltage crossings (Zero Voltage Switching). These switching techniques help to reduce the switching losses and EMI. However, to do so, an extra voltage transformer or winding is usually needed to detect the zero voltage crossings across the capacitor C. The detected information is used by the controller to drive the switches S1 and S2 and special gate drive circuits are usually required. The start up of this form of inverter is particularly difficult, requiring a complex controller.
Therefore, apart from the semiconductor switches and tuning capacitors, there are many magnetic components required for the conventional push pull current fed resonant inverter.